Saturn’s approach to equinox reveals new detail in rings

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 15 June, 2009

Towering vertical structures in Saturn’s rings have been discerned for the first time by Cassini as the giant planet approaches equinox.

Never-before-seen vertical structures created by the tiny moon Daphnis cast long shadows across the rings of Saturn as the planet approaches equinox. Image: NASA/JPL/Space Science Institute.

Equinox defines the exact moment when the Sun is seen directly overhead at noon at a planet’s equator, and occurs twice a year. For Saturn the wait is long, occurring every 15 Earth years. The wait has been well worth it though, for Cassini has detected the long shadows cast across the expanse of rings by never-before-seen structures as the giant planet approaches equinox this August. The observations also confirm that small moons in narrow ring gaps can have significant and complex effects on the edges of their gaps.

In one example, the eight kilometre wide moon Daphnis orbits within the 42 kilometre wide Keeler Gap in Saturn’s outer A ring, where its gravitational pull perturbs the orbits of the particles forming the gap’s edges, especially when the moon’s eccentric orbit brings it very close to the gap edges, resulting in time-variable patterns being produced. The new analysis also shows that when such a moon has an orbit inclined to the ring plane, as does Daphnis, the time-variable edge waves also have a vertical component to them.

Vertical structures created by Saturn's small moon Daphnis cast long shadows across the rings in this dramatic image taken as the planet approaches its mid-August 2009 equinox. Image: NASA/JPL/Space Science Institute.

Cassini scientists estimate that the wave heights extend a staggering 1.5 kilometres above Saturn's ring plane, making them twice as high as previously known vertical ring structures, and as much as 150 times as high as the rings are thick. “We thought that this vertical structure was pretty neat when we first saw it in our simulations,” says Cassini Imaging Scientist John Weiss. “But it’s a million times cooler to have your theory supported by such gorgeous images. It makes you suspect you might be doing something right.”

The team also came up with a refinement to a theory used since the Voyager missions of the 1980s to infer the mass of moons embedded in ring gaps based on how much the moons affect the surrounding ring material. The new analysis suggests that an embedded moon in a very narrow gap can have a smaller mass than that inferred by earlier techniques.

The vertical structures rise high above the planet's otherwise flat, thin disk of rings. Image: NASA/JPL/Space Science Institute.

One of the prime future goals of the imaging team is to scour the remaining gaps and divisions within the rings to search for the moons expected to be there. “It is one of those questions that have been nagging us since getting into orbit: ‘Why haven’t we yet seen a moon in every gap?’” says Cassini imaging team leader Carolyn Porco. “We now think they may actually be there, only a lot smaller than we expected.”

The search for ring material extending out from the ring plane has been a major goal of the imaging team during Cassini’s extended two-year Equinox Mission, which will see continued examination of Saturn as it moves through its August 2009 equinox, as well as further study of large moons Enceladus and Titan.

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